In so-called recuperative heat exchangers of the type described above, a stream of moist, warm so-called exhaust air, as from a room or building, is cooled by a cool stream of fresh air or the like, the two streams exchanging heat without coming in direct contact. As the cooling stream of fresh air or similar cooling medium absorbs heat from the exhaust air, and as the temperature of the latter falls, moisture condenses on the heat exchange surfaces of the exhaust air channels. Especially in the colder season, this often results in the formation of frost or ice in the channels through which the exhaust air passes, thus greatly reducing the efficiency of the heat exchanger and in severe cases blocking the channels.